A light diffusing element is widely used in illumination covers, screens for projection televisions, surface-emitting apparatus (for example, liquid crystal display apparatus), and the like. In recent years, the light diffusing element has been used for enhancing the display quality of the liquid crystal display apparatus or the like and for improving a viewing angle characteristic, for example. As the light diffusing element, for example, there is proposed a light diffusing element in which fine particles are dispersed in a matrix such as a resin sheet (see, for example, Patent Literature 1). In such light diffusing element, most of incident light scatters forward (output plane side), whereas a part thereof scatters backward (incident plane side). As a refractive index difference between each of the fine particles and the matrix becomes larger, diffusibility (for example, a haze value) increases. On the other hand, as the refractive index difference becomes larger, backscattering increases. When the backscattering is significant, in the case where the light diffusing element is used in a liquid crystal display apparatus, its screen becomes whitish at the time of entrance of external light into the liquid crystal display apparatus, and hence it is difficult to display a video or an image having contrast.
As means for eliminating the backscattering as described above, there has been proposed a light diffusing element obtained by dispersing, in a resin, refractive index gradient fine particles, such as so-called gradient index (GRIN) fine particles, in each of which a refractive index continuously changes from a center of the fine particle toward an outer side (see, for example, Patent Literature 2). However, the GRIN fine particles are insufficient in productivity owing to their manufacturing process, which is more complicated than that of general fine particles, and hence are not practical.
In addition, as means for continuously changing the refractive index in the light diffusing element including the GRIN fine particles, there has been proposed a technology involving allowing a precursor of a matrix resin component (e.g., a monomer) to permeate fine particles before polymerizing a matrix resin component (see Patent Literature 3). However, even in the case of such technology, in order to obtain high-haze light diffusibility, the permeation of the precursor of a matrix resin component requires a long period of time or requires heating at high temperature, and hence there is still room for improvement in terms of productivity.